The present invention relates to the production of a fine-grained vinyl chloride suspension polymer and its use as a viscosity-lowering agent in polyvinyl chloride plastisol processing.
Polyvinyl chloride plastisols or polyvinyl chloride pastes are generally understood to mean dispersions of fine polyvinyl chloride powders in plasticizers. Suitable paste-forming types of polyvinyl chloride include emulsion polymers and, in particular, also microsuspension polymers.
These polyvinyl chloride products are obtained after polymerization in latex form and are processed, generally, by spray-drying into agglomerated secondary particles (plastisol grades). During plastisol processing, these agglomerates disintegrate predominantly into primary particles. The degree of such disintegration and the particle size distribution of the primary particles determine the flow properties of the paste.
Polyvinyl chloride plastisols or pastes are utilized primarily as spread-coating pastes, as dip-coating pastes and casting pastes, and as molding pastes for a great variety of finished articles.
In general, pastes of low viscosities are desirable for the manufacture of finished articles low in plasticizer. It is known for this purpose to lower the paste viscosity by the addition of fine-grained suspension polyvinyl chloride products (so-called extender polyvinyl chloride) which are not paste-forming and which are dissolved only at the gelling temperature.
According to German Patent No. 1,645,668, it is possible to produce such suspension polymers for lowering the viscosity of polyvinyl chloride plastisols using methylhydroxypropylcelluloses having a viscosity of 50-500 mPa.s at 20.degree. C. in a 2% by weight solution, as the only suspension stabilizer, and using monomer-soluble catalysts.
However, as can be seen from Comparative Example A (in accordance with German Patent No. 1,645,668 with a methylhydroxypropylcellulose as the suspension stabilizer, having a viscosity of 100 mPa.s at 20.degree. C. in a 2% by weight solution), the thus-produced polymers contain a considerable proportion of coarse grains having particle sizes above 100 .mu.m. This causes an undesirable sedimentation of the coarse proportion and, therefore, difficulties in processing. Furthermore, the coarse proportion results in an undesirably rough surface, especially when applying very thin coatings.
As can be seen from Comparative Example B (also according to German Pat. No. 1,645,668), finely divided polyvinyl chloride polymers can be prepared by suspension polymerization in the presence of a methylhydroxypropylcellulose having, in a 2% by weight solution, a viscosity of 50 mPa.s at 20.degree. C. The thus-produced polymer is suitable, on account of its fineness and its minimum proportion of coarse particles (&gt;100 .mu.m), for thin coatings and does not tend toward sedimentation. However, due to its very high granular porosity, the resultant product provides inadequate lowering of paste viscosity. It does not satisfy present requirements under practical conditions.